The present invention relates to an improved process for the preparation of carbodiimide modified organic isocyanate, preferably polyphenylmethane polyisocyanates, and most preferably diphenylmethane diisocyanates. This process comprises (1) neutralizing acidic impurities in an organic isocyanate, (2) partially carbodiimidizing isocyanate groups of the neutralized organic isocyanate, (3) terminating the carbodiimidization reaction.
Carbodiimidization of isocyanates is known and described in, for example, U.S. Pat. Nos. 2,853,473, 4,937,012, 5,202,358, 5,610,408, 6,120,699, 6,362,247, and 6,489,503, and in EP 193,787. Carbodiimidization of isocyanates is desirable to provide storage stable liquids. Liquids are easier to pump and less expensive to transport than fused solids or slurries. The liquids are homogeneous compositions as supplied without the need to homogenize as with slurries or fused solids. In the production of polyurethanes, a liquid can be added easily by weight or volume and combined with suitable co-reactants at room temperature. This is safer than using the materials at elevated temperature and the corresponding higher vapor pressure of the heated materials.
Methods for improving stability and/or reactivity of polyisocyanates are also known and described in the art. See U.S. Pat. Nos. 3,793,362, 5,342,881, 5,726,240, 5,783,652 and 6,528,609. Most of these patents disclose blending or mixing an organic polyisocyanate with an epoxide or other compound.
Many of these methods describe improving the reactivity of polymer MDI or adducts prepared from MDI that initially have adicity values that well exceed 25 ppm as measured using ASTM D 5629. By comparison, the refined starting materials described in the present invention typically have acidity values well under 25 ppm. Due to the extremely low levels of highly efficient catalyst used in the preparation of the carbodiimides described in the present invention and to the sensitivity of these catalysts to acidic impurities, it is necessary to remove even this low amount of acidity.
Normally, the acidity of the isocyanate can be lowered to levels below 25 ppm by distillation. Depending on the efficiency of the columns used in the distillation process, these levels can be reduced to a range of 1–10 ppm. Trace levels of hydrogen chloride or hydrolysable chloride can be further removed by heating the isocyanate and passing an inert gas through the materials during distillation as in U.S. Pat. No. 3,516,950.
U.S. Pat. Nos. 4,814,103, 6,127,463 and 6,166,128 disclose that the color of various organic polyisocyanates can be stabilized and/or reduced by the addition of epoxides alone or in combination with hindered phenols.
Copending application Ser. No. 10/870/126, filed in the U.S. Patent and Trademark Office on Jun. 17, 2004, the same day as the present application, and which is commonly assigned, relates to TDI prepolymers with improved processing characteristics. These TDI prepolymer compositions comprise from about 95 to about 99.99% by weight of a prepolymer of toluene diisocyanate, and from about 0.01 to about 5% by weight of an epoxide having an epoxide equivalent weight of from about 44 to about 400. The prepolymer of TDI comprises the reaction product of toluene diisocyanate containing from about 60 to about 100% by weight of the 2,4-isomer and from about 0 to about 40% by weight of the 2,6-isomer, and an isocyanate-reactive component having a functionality of from about 1.5 to about 8 and an OH number of from about 14 to about 1870.
Advantages of the present invention include lower color of the carbodiimide polyisocyanate product due to quicker processing to form the carbodiimide. The resulting products can be produced using lower levels of carbodiimidization catalyst so that the stability of the final product is improved. Also, the lower amount of catalyst requires less catalyst stopper which is advantageous since catalyst stopper can cause yellowing of the final product.